Dissertation Research: Phylogeny of Aplacophora and Polyplacophora and Evolution of Molluscan Calcareous Hard Parts

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0073312Knoll and OkusuGraduate student Akiko Okusu, under the direction of Dr. Andrew Knoll, Harvard University, is studying the phylogenetic relationships and the evolution of calcareous hard parts of aplacophoran and polyplacophoran molluscs using integrated data from DNA sequences, morphology, and embryology. Aplacophorans are worm-like deep-sea molluscs that have no shells but are covered by spicules made of the calcium carbonate mineral aragonite. Polyplacophorans (chitons) are flat elongate molluscs with eight dorsal shell-plates imbricated within a spicule-embedded mantle girdle. It has been postulated that aplacophorans are closely related to polyplacophorans, that the two groups represent the most basal groups of molluscs (based on their relatively simple features), and that their aragonite spicules reflect a more "primitive" type of calcium carbonate deposition than that found in other molluscs. However, the evolutionary relationships of the Aplacophora and the Polyplacophora are unresolved. The lack of a well-supported phylogeny calls into question hypotheses of evolutionary changes in molluscan form and mineral deposition. Molecular sequence data from highly conserved nuclear coding genes, EF1-alpha and POL-II, will be used to construct a phylogenetic framework for 40 taxa including representative species from major molluscan classes and outgroup species from the Sipuncula and Annelida. Spicule development and morphology in these groups will be analyzed within the phylogenetic framework in order to assess homology and pattern of evolutionary change. The results will have a major impact on our understanding of the origin and evolution of molluscs.